Identification of a biomanipulation‐driven regime shift in Lake Vesijärvi: implications for lake management

Summary The biomanipulation of Lake Vesijärvi in southern Finland putatively changed the lake ecosystem resulting in a clear‐water state, but the dynamics behind the observed change remained unverified. We used a general analytical approach to identify abrupt transitions in the long‐term monitoring data of Lake Vesijärvi. Methods included the identification of breakpoints in chlorophyll a (response variable), in total phosphorus (TP; key driver) and in large‐bodied cladocerans (feedback mechanism of biomanipulation). In addition, we characterised the frequency distribution and temporal variance of the response variable, as well as the interaction between TP and chlorophyll a . According to long‐term records, biomanipulation resulted in two distinct states: (i) a eutrophic state, where chlorophyll a and TP were linearly correlated and (ii) a mesotrophic state, where the key driver and response variables were not significantly correlated. The changed relationship between the key driver and response variables, together with the increase in the body size of cladocerans, suggested a hysteresis‐type transition phase. However, the frequency distribution and the temporal variance of chlorophyll a agreed also with a threshold‐type of transition. Zooplankton seemed to be more important in maintaining the clear‐water state than previously reported. Our results highlight the role of long‐term monitoring data and the characterisation of abrupt transitions in ecosystem interactions as tools for sustainable management of lake ecosystems.